A numerical model of cardiovascular fluid mechanics during external cardiac assist

Abstract

External cardiac assist (ECA) is a noninvasive, counterpulsative method that aims to provide temporary aid to the failing heart through increased diastolic augmentation and decreased systolic after load. These counterpulsative benefits are achieved by sequentially inflating cuffs on the calves, thighs. and buttocks 180 degrees out-of-phase with the left ventricle. Previous clinical and computational studies have demonstrated the efficacy of ECA is highly dependent on the selection of the control scheme parameters for the device. These control parameters include: the maximum external pressure applied by each cuff, the starting time of cuff inflation and deflation, the time between adjacent cuff inflations, and the total time to inflate and deflate the cuffs from atmospheric to maximum pressurization and vice versa. Optimization of these control parameters, however, necessitates a complete understanding of the hemodynamics created by ECA. Clinical ECA devices used currently have been designed without the benefit of this understanding. Thus, a numerical model has been developed that simulates cardiovascular hemodynamics during the operation of an ECA device. The model includes a 28-element distributed arterial system with boundary conditions simulating the left ventricle, bifurcations, and peripheral arterial vessels. Some of the hemodynamic phenomena studied in the model include: the collapse and subsequent emptying of arteries as external pressure is applied, the refilling of arteries as external pressure in the cuffs is released, and the changes in aortic pressure, cardiac output, and blood flow distribution resulting from operation of the ECA device. By quantitatively studying these hemodynamic phenomena, the parameters governing the application and control of the ECA device were systematically varied and an optimum set of control parameters was determined.